The present invention relates to a position detector for a movable body. More particularly, the present invention pertains to a device for detecting the position of a piston in a fluid cylinder used in industrial vehicles.
A typical forklift has a fork for lifting and lowering a cargo. Some forklifts are equipped with an automatic controller for lowering or lifting the fork to a predetermined position. The automatic controller requires a fork height sensor to continuously detect the height of the fork.
Reel-type fork height sensors are known in the art. A reel type sensor includes a wire, a reel for winding the wire and a rotation sensor such as a potentiometer. One end of the wire is connected to an inner mast. The rotation sensor detects rotation of the reel. The height of the fork is detected based on the rotational position of the reel.
However, the wire is exposed. Therefore, when the forklift is operated, the wire can be damaged by contact with foreign objects, which may cut the wire or damage the potentiometer. The reliability of the sensor is thus low.
To solve the above problem, fork height detectors using an ultrasonic sensor have been introduced. This fork height detector includes a lift cylinder for lifting and lowering a fork and an ultrasonic sensor located in the lift cylinder. The ultrasonic sensor detects the location of a piston in the lift cylinder. The height of the fork is based on the detected position of the piston. Specifically, the lift cylinder includes a cylindrical housing, a piston accommodated in the housing and an ultrasonic element. The ultrasonic element is located at the bottom of the cylindrical housing. The ultrasonic element produces ultrasonic waves to the end surface of the piston and receives the reflected ultrasonic waves. The distance between the element and the piston, or the position of the piston, is calculated based on the traveling time of ultrasonic waves, or the time from when ultrasonic waves are output to when reflected waves are received. The height of the fork is calculated based on the detected piston position. Unlike reel type sensors, the functional part of the ultrasonic height detector is not exposed. Therefore, the height detector is less vulnerable to damage, which improves reliability.
However, the detection accuracy of ultrasonic sensors is low. Ultrasonic waves from an ultrasonic element are transmitted through oil in a cylindrical housing, or oil chamber. As shown in FIG. 6, the transmission speed of ultrasonic waves (speed of sound) varies in accordance with the temperature of the oil. As a result, when the piston stays at a certain position, the position detected by the ultrasonic sensor changes in accordance with temperature of the oil as shown in FIG. 7. The temperature of the oil in the lift cylinder is greatly varied by the ambient temperature and the duration of forklift operation. The temperature changes of the oil lower the detection accuracy of the fork height detector.
Therefore, the detected position of the piston includes an error due to the temperature of the oil, which prevents the accurate position of the fork from being detected. Further, the low accuracy of the height detection lowers the accuracy of the fork control.
In order to produce ultrasonic waves from the ultrasonic element, an oscillation signal is sent to an ultrasonic oscillator in the ultrasonic sensor. Once oscillated, the ultrasonic element does not stop oscillating immediately after the oscillation signal is stopped. While being dampened, the ultrasonic oscillation continues for a certain time. This called reverberation.
As shown in FIG. 11, reverberation remaining in the ultrasonic element generates a voltage signal. Therefore, if the reflection of an ultrasonic wave produced by the ultrasonic oscillator is received by the ultrasonic element while there is reverberation, the reflected wave is mixed with the reverberation. That is, the reflected wave and the reverberation are not distinguished. This lowers the reliability of the detection value. Using a fork height sensor having such an ultrasonic element causes a problem. When the fork is at the lowest position, the piston of the lift cylinder is extremely close to the ultrasonic element. At this time, an ultrasonic wave reflected by the piston can interfere with a subsequent ultrasonic wave produced by the ultrasonic element, which prevents the position of the piston, or the height of the fork, from being accurately detected.